Polyethylene (PE) is classified as a “thermoplastic”. A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polyethylene liquefy, which allows them to be easily molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once.
Different types of polyethylene exhibit wide variability in their crystalline structures. The less crystalline a plastic is (i.e. the more amorphous a plastic is), the more it demonstrates a tendency to gradually soften (i.e. they have a wider range between their glass transition temperature and their melting point). Polyethylene is a homopolymer in that it is composed of a single monomer constituent which is H2C=CH2 in ethylene.
Polyethylene has an extremely large range of applications depending on the particular type. It is one of the most widely produced plastics in the world. Polyethylene is commonly categorized into one of several major compounds of which the most common include LDPE, LLDPE and HDPE. High Density Polyethylene (HDPE) is a strong, high density, moderately stiff plastic with a highly crystalline structure. It is frequently used as a plastic for milk cartons, laundry detergent, garbage bins, and cutting boards
High density Polyethylene can be produced by three types of processes using Ziegler-Natta or Phillips-Type Catalyst:
1. Slurry Process:
The catalyst is diluted with any liquid hydrocarbon (e.g. hexane). A mixture of ethylene and hydrogen is passed under pressure through this slurry, after which the monomer (ethene) is polymerized to form polyethylene. The polymer-solvent slurry is evaporated to remove the solvent. Water vapour and nitrogen is then passed through the separated polymer to destroy the active sites of the catalyst, thus leaving behind HDPE.
2. Solution Process:
A mixture of ethylene and hydrogen is passed through a solution containing the Zieglar catalyst and liquid hydrocarbon under pressure. The remaining procedure is similar to the slurry process.
3. Gas Phase Process:
In a fixed bed reactor, a hydrogen and ethylene mixture is passed through a Phillips-Type catalyst. Ethylene, which is suspended in the gas, will polymerise to form HDPE. The polymer powder obtained is separated and shaped as required.
HDPE is used for making bags, storage bottles, tanks, etc due to its high stiffness property. Most of the bottles are made using the process of blow molding process.
HDPE is used for making pipes for gas/liquid/sewage industries due to its chemical inertness, corrosion resistance, light weight (compared to metal pipes), and high strength properties.
HDPE is used to make mortars for the pyrotechnic system, and is a replacement for PVC/steel mortars, because it is much safer.